A force-measuring device normally includes a force-receiving part, a force-transmitting part, a force-measuring cell, and a device for the processing of the measuring signals. To perform a force measurement, the force is received by means of the force-receiving part and transmitted by way of the force-transmitting part to the force-measuring cell, where it acts as input force to the force-measuring cell. For example in a weighing device, the force to be measured is constituted by the weight force of the weighing object which is applied to the force-receiving part (in the form of a weighing pan) and transmitted through the force-transmitting part (in the form of a linkage) to act on the force-measuring cell, the so-called weighing cell.
The force-measuring cell is a mechanical-to-electrical transducer which converts the input force into an electrical measuring signal. This measuring signal, which corresponds to the input force, is transmitted to a processing unit, where it is processed and evaluated. The results of the processing are passed on in the form of measurement values to an indicator unit or to a further processing unit, for example a higher-level computer or a system controller.
In order to ensure a consistently high accuracy, it may be necessary from time to time to perform a corrective procedure called calibration. This entails that a force of a predefined magnitude is applied to the force-measuring cell, so that the force-measuring cell generates a measuring signal which corresponds to the predefined force and is transmitted to the processing unit. Based on the correlation between a predefined force and the measurement values derived from it, it is possible to take appropriate corrective measures, for example to adjust the calculation parameters in the processing unit.
The functions of generating the predefined force and transmitting this force to the force-measuring cell are often performed by a calibration device. During the process of a calibration, the calibration device or at least part of it, for example a calibration weight, is coupled to the force-measuring cell through a force-transmitting connection, and the force that is thereby generated is transmitted to the force-measuring cell. At the completion of the calibration cycle, the coupling is disconnected so that when the force-measuring cell is working in its normal measuring mode, it is separated from the calibration device.
A dead-weight loading machine is disclosed in DE 10 2007 036 214 A1 which serves for the calibration of a wide variety of force-measuring devices, for example in a national standards laboratory. The machine described there includes a pressure plate which can support a force transducer that is to calibrated, as well as a rack with predefined weights which can be connected from the outside to the force transducer either individually or in any combination. The loading and lifting of the weights occurs by means of a pneumatic actuator element system, wherein the latter can in particular have the capability of being controlled automatically. In such an arrangement, the electrical output of the force transducer, for example a force-measuring cell, is connected to the control system. The automatic control is performed interactively by means of a menu-driven application-specific software program. Prior to the start of the calibration, the user is required to enter parameters concerning the force-measuring cell as well as the calibration procedure. The selection of the weights as well as the evaluation of the results and the issuance of a calibration reports can likewise take place automatically.
Calibration devices which are assigned to a force-measuring device in order to calibrate the latter when needed are known in the existing state of the art. For example, CH 676 750 A5 discloses a calibration device with a predefined calibration weight which can be lowered by means of a lifting device onto a carrier that is coupled to the force-measuring cell, whereby the calibration weight is coupled to the force-measuring cell. At the beginning of the calibration cycle, the calibration weight is moved vertically downward by means of an electric motor and a rotating screw spindle and then raised up again after the calibration has been completed. The lifting device can be configured as an autonomous unit and can be adapted for use with calibration weights of different sizes. However, the capabilities of this calibration device for adaptation to different force-measuring devices or to specific requirements are limited. Yet, it remains a desirable attribute for a calibration device to be flexible and simple to adapt to the requirements of the applications in which it is used and of the force-measuring device to which it is assigned.
It is therefore one object of the present invention to provide a calibration device for a force-measuring device which can be adapted in a simple manner to different requirements. A further goal is to achieve a simple and cost-effective configuration and operation of the force-measuring device, and at the same time to meet stringent requirements in regard to measurement accuracy and stability.